That is correct. Decades ago, my cousin was into recreational lock-picking, and it turns out the bristles are perfect for machining into such tools.

EDIT - whoops, I stand corrected, metal or plastic.

Recommended by the MIT guide to lock picking - https://www.lysator.liu.se/mit-guide/MITLockGuide.pdf page 44

They are no that great because they rust. Windshield wiper blades are a much better metal. And you can find tons of them just going to an auto store parking lot after a rain.

Quite often, yes! I sometimes collect them for making lockpicks and other small tools (a common practice!)

The ones I've seen are either metal (black ones) or very hard plastic (blue).

>very hard plastic (blue).

mmm... microplastics